Charles Hartshorne was a leading proponent of process philosophy and particularly of process theology. Probably no single philosopher can be more closely identified with the promotion of panentheism as a rational theological scheme.

He was born to a Pennsylvania reverend in 1897 and followed World War I service as an Army hospital orderly with a succession of ever-more rarified Philosophy degrees from Harvard University.

Hartshorne approached theology as an exercise in logic. He examined and categorized over thirty theological schemes (such as theism, polytheism, pantheism, pandeism, and emanationism), and applied logical formulas designed to winnow away the less defensible schemes. Among the many innovations that Hartshorne would reach was the conception of a God that was not perfect in all ways (as previous philosophers had argued) precisely because there are some ways in which perfection is meaningless. Can God paint a perfect painting? (assuming that the people who see the painting do not know it is by God, and God does not alter their thoughts to make them think it perfect if they would not be so inclined). Well, no because different people have different tastes, so even if some thought it was perfect, others might think it lacked something, or had too much of something, though a truly "perfect" painting would be one that everyone would agree was perfect. Is it within God's power to commit a perfect sin? Can God be perfectly evil? If God is perfectly good, then it would seem these things are beyond the power of God!

Hartshorne particularly thought that it was a mistake to describe God as incapable of change. He wrote of this conception that "God, being perfect, cannot change (not for the better, since "perfect" means that there can be no better; not for the worse, since ability to change for the worse, to decay, degenerate, or become corrupt, is a weakness, an imperfection). The argument may seem cogent, but it is so only if two assumptions are valid: that it is possible to conceive of a meaning for "perfect" that excludes change in any and every respect and that we must conceive God as perfect in just this sense." Hence Hartshorne referred to the arguments for God's omnipotence as a "theological mistake."

Hartshorne coined a model called "AR"—"absolute perfection in some respects, relative perfection in all others"—to convey the idea that God is indeed perfect, but to acknowledge that there are many characteristics not amenable to such a description: God expresses absolute perfection along some dimensions, relative perfection along others. God may not have the power to create a "perfect" painting, but his ability to create a painting would surpass that of any human who could ever lift a brush, even if there would inevitably be some who did not care for the work. So it is, Hartshorne proposes, with power and, most importantly, knowledge. God is relatively all-powerful, in that God has complete power over everything that exists in the Universe (or over everything that was to become the Universe before the Universe existed); and similarly God has complete knowledge of everything that exists in the Universe (or, again, over everything that was to become the Universe before the Universe existed).

Hartshorne particularly considered five models: theism, pantheism, deism, pandeism, and panentheism. He rejected pantheism in favour of pandeism because he found a "theos" to be necessary, but in turn rejected both pandeism and deism in favour of panentheism; he accepted the pandeistic argument that the Universe was a manifestation of God, he was not prepared to accept a God that was not also separate from and above the Universe. He wrote that: "Just as AR is the whole positive content of perfection, so CW, or the conception of the Creator-and-the-Whole-of-what-he-has-created as constituting one life, the super-whole which in its everlasting essence is uncreated (and does not necessitate just the parts which the whole has) but in its de facto concreteness is created-—this panentheistic doctrine contains all of deism and pandeism except their arbitrary negations."

Hartshorne concludes: "ARCW, or absolute-relative panentheism, is the one doctrine that really states the whole of what all theists, if not all atheists as well, are implicitly talking about." However, Hartshorne’s rejection of deism and pandeism is conditional, applicable only if their "negations" are in fact arbitrary. The negations of deism (i.e. God's inactivity in the Universe) are generally arbitrary indeed. It is, naturally, possible to articulate theories of pandeism with arbitrary negations, but others exist in which the negations are not arbitrary at all, but are instead absolutely necessary to account for the nature of the Universe, and the motivation of the Deus in carrying forward with exactly such a creation. However, Hartshorne's conception of the perfection of God required not only immanence (and the evolution of God itself, but also God's active involvement in the world. Thus, Hartshorne ultimately embraced panentheism as his final conclusion of the nature of God.

Hartshorne lived to be a hundred and three years old, and because he was born in the 1890s and died in the year 2000, he is one of the few people whose life touched three different centuries. Towards the end of his life he was asked which among the dozens of books that he had written was his greatest. Eschewing all of his philosophical works, he identified a book he had written on bird songs.